It's likely a good deal depending on the condition and configuration of the gun. It's a good price for an 870 Skeet or a gun with a vent rib and choke tubes, less so for a plain barrel, field grade with a fixed choke.
Shell availability depends on where you are. You won't find them at Wally World or Canadian Tire but most reasonably stocked gun shops will have or can get them them. They are pricey and will be more expensive than 12 or 20 gauge. In factory form there isn't a lot of variety of shells with only light target or small bird hunting loads. No slugs or buckshot. We 28 gauge fans typically reload.
But for small bird hunting, clay shooting and general fun with very little recoil the 28 is hard to beat.